KR20210142920A - Epoxy fire resistant coating composition - Google Patents

Abstract

The present invention provides an epoxy fire resistant coating composition which satisfies long-term fire resistance performance and is excellent in coating film strength and elasticity at the same time. The epoxy fire resistant coating composition of the present invention includes an epoxy resin, a reactive diluent, a flame retardant, and a curing agent, and the reactive diluent includes a trifunctional or higher reactive diluent and a bifunctional or lower reactive diluent.

Description

EPOXY FIRE RESISTANT COATING COMPOSITION

The present invention relates to an epoxy fire resistant paint composition having excellent fire resistance performance and excellent coating film strength and elastic modulus.

Oil fires occurring in offshore structures and plants are characterized by a rapid rise in temperature to about 945 ℃ within 5 minutes in case of fire. Therefore, in the field where there is a possibility of oil fire, a paint having excellent fire resistance performance is used, and a curing type epoxy fire resistant paint is mainly used.

For example, U.S. Patent No. 4,529,467 discloses that when the paint expands in case of fire, zinc (Zn) forms a foaming layer in the char, and the formed foaming layer can improve heat insulation and adhesion to the substrate. paint is being started. However, when zinc is used in excess, the coating film may not expand and thus the insulation performance may be deteriorated.

US Patent Application No. 1997-999536 discloses a fire resistant epoxy paint that exhibits fire resistance while reducing the amount of paint used by lowering the density of the coated film using hydrophobic fumed silica. However, the paint contains a high content of the phosphorus-based flame retardant TPP (Triphenyl phosphate), which is a low-molecular material and improves initial foaming because it is vaporized at a lower temperature than a cured epoxy resin, but when the TPP content is large, the gas toxicity increases, There is a problem that the long-term fire resistance performance is lowered.

On the other hand, paints to improve long-term fire resistance performance by preventing cracks in the carbonized layer by providing flexibility to the foam layer have been proposed.

The present invention provides an epoxy fire resistant paint composition that satisfies long-term fire resistance performance and has excellent coating film strength and elasticity at the same time.

The epoxy fire resistant paint composition of the present invention includes an epoxy resin, a reactive diluent, a flame retardant and a curing agent, and the reactive diluent includes a trifunctional or higher reactive diluent and a bifunctional or lower reactive diluent.

The epoxy fire resistant paint composition of the present invention provides flexibility to the foam layer to prevent cracks from occurring in the carbonized layer, thereby satisfying long-term fire resistance performance of 3 hours or longer. In addition, the epoxy fire resistant paint composition of the present invention is excellent in coating film strength and elastic modulus, and the content of TPP is low to reduce gas toxicity.

Hereinafter, the present invention will be described in detail. However, it is not limited only by the following content, and each component may be variously modified or selectively mixed as needed. Accordingly, it should be understood to include all modifications, equivalents and substitutes included in the spirit and scope of the present invention.

The epoxy fire resistant paint composition according to the present invention includes an epoxy resin, a reactive diluent, a flame retardant and a curing agent, and the reactive diluent includes a trifunctional or higher reactive diluent and a bifunctional or lower reactive diluent. The epoxy fire resistant paint composition of the present invention may further include a foaming agent, an acid catalyst, a fiber, and additives commonly used in the art.

As used herein, "weight average molecular weight" is measured by a conventional method known in the art, for example, it can be measured by a GPC (gel permeation chromatograph) method. A functional group such as “acid number” is measured by a method well known in the art, and may be determined by, for example, titration. "Viscosity" is measured by a conventional method known in the art, and can be measured, for example, using a Brookfield viscometer.

epoxy resin

In the epoxy fire resistant paint composition of the present invention, the epoxy resin serves to promote adhesion to a target object and enhance durability of the cured coating film. In addition, it changes the coating film to a fluid state when exposed to high-temperature heat in the event of a fire so that the coating film can properly expand when gas is generated, and serves as a skeleton of the foamed carbonized layer.

As the epoxy resin, a bisphenol A type epoxy resin, a bisphenol F type epoxy resin, a flame retardant epoxy resin, a novolak type epoxy resin, a polyfunctional amine epoxy resin, a cycloaliphatic epoxy resin, or a mixture thereof may be used. It is not limited. For example, at least one selected from a bisphenol A type epoxy resin and a bisphenol F type epoxy resin may be used.

The weight average molecular weight of the epoxy resin may be 150 to 900 g/mol, for example, 200 to 800 g/mol, in another example 360 to 400 g/mol. If the weight average molecular weight of the epoxy resin is less than 150 g/mol, sagging may occur and the coating film may flow down. can be inferior

The epoxy equivalent of the epoxy resin may be 75 to 450 g / eq, for example 100 to 400 g / eq, in another example 180 to 200 g / eq. If the epoxy equivalent of the epoxy resin is out of the above range, the coating may not be properly formed because curing of the paint may not proceed sufficiently.

The epoxy fire resistant paint composition of the present invention may include 10 to 25% by weight of the epoxy resin, for example 15 to 23% by weight, based on the total weight of the composition. If the epoxy resin content is less than 10% by weight, the adhesion of the coating film may be reduced, and if it exceeds 25% by weight, the foaming rate of the coating film may be inhibited in case of fire, so that sufficient fire resistance performance may not be exhibited.

reactive diluent

In the epoxy fire resistant paint composition of the present invention, the reactive diluent reacts with the curing agent to reduce the paint viscosity while participating in the film formation, and serves to impart flexibility to the paint film. The reactive diluent includes a trifunctional or higher functional reactive diluent and a bifunctional or lower reactive diluent.

As the trifunctional or higher reactive diluent, a reactive diluent having three or more reactive groups reacting with the curing agent, for example, 3 to 6, or 3 or 4 reactive diluents may be used. The trifunctional or more reactive diluent may be a trifunctional or more functional (meth)acrylate monomer.

Non-limiting examples of the trifunctional or higher (meth)acrylate monomer include trifunctional (meth)acrylate monomers such as trimethylolpropane tri(meth)acrylate and pentaerythritol tri(meth)acrylate; tetrafunctional (meth)acrylate monomers such as pentaerythritol tetra(meth)acrylate and ditrimethylolpropane tetra(meth)acrylate; There are dipentaerythritol poly(meth)acrylates, and these may be used alone or in combination of two or more. For example, the trifunctional or more (meth)acrylate monomer may be a trifunctional (meth)acrylate monomer, such as trimethylolpropane tri(meth)acrylate.

The trifunctional or more reactive diluent has an epoxy equivalent weight of 70 to 130 g / eq, for example 90 to 110 g / eq, and a viscosity of 60 to 140 cps (25 ° C.), for example 80 to 120 cps (25 ° C.) can be When the epoxy equivalent of the trifunctional or higher reactive diluent is less than 70 g/eq, the flexibility of the cured coating film is low, and deterioration of the physical properties of the coating film may occur, such as cracks occurring at low temperatures, and when it exceeds 130 g/eq, the coating film hardness is In case of external impact, the coating film may be easily damaged and the long-term durability of the coating film may be inferior.

The bifunctional or less reactive diluent includes phenyl glycidyl ether, alkyl glycidyl ether, glycidyl ester of versatic acid, olefin epoxide, 1,6-hexanediol diglycidyl ether, neopentyl glycol di Glycidyl ether, trimethylolpropane triglycidyl ether, alkylphenyl glycidyl ether, such as methylphenyl glycidyl ether, ethylphenyl glycidyl ether, propylphenyl glycidyl ether, neodecanoic acid 2,3-epoxy propyl ester and the like can be used. The above-mentioned components may be used alone or two or more may be used in combination.

The bifunctional or less reactive diluent has an epoxy equivalent weight of 100 to 350 g/eq, for example 130 to 250 g/eq, and a viscosity of 5 to 50 cps (25° C.), for example 5 to 30 cps (25° C.) ) can be If the epoxy equivalent of the bifunctional or less reactive diluent is less than 100 g/eq, the diluent is eluted to the surface of the coating film at high temperature, and adhesion between layers may be inhibited during topcoating. As the carbonized layer of the fire resistant paint used is easily broken and the flame is exposed through the cracks, the fire resistance performance may be greatly reduced.

In the present invention, the mixing ratio of the trifunctional or higher reactive diluent and the bifunctional or lower reactive diluent may be 80: 20 to 20: 80, for example, 70: 30 to 30: 70 by weight. Based on the total weight of the trifunctional or higher reactive diluent and the bifunctional or lower reactive diluent, if the content of the bifunctional or lower reactive diluent is less than 20% by weight, large pores (diameter 1cm or more), and the more large pores, the easier the carbonized layer is cracked during the foaming process, so flames can easily penetrate through the cracks and the fire resistance performance may drop sharply. On the other hand, when the content of the reactive diluent less than or equal to bifunctional based on the total weight of the reactive diluent having more than trifunctionality and the reactive diluent having less than bifunctionality is more than 80% by weight, the hardness of the coating film is decreased and the coating film is easily broken during external impact. The durability of the coating film may be inferior.

The epoxy fire resistant paint composition of the present invention may include 2 to 20% by weight of the reactive diluent, for example 2 to 10% by weight, based on the total weight of the composition. For example, the epoxy fire resistant paint composition of the present invention is based on the total weight of the composition, 1 to 10% by weight of the trifunctional or more reactive diluent, such as 1 to 5% by weight, and 1 to 10% by weight of the bifunctional or less reactive diluent , for example, 1 to 5% by weight. When the content of the reactive diluent is less than the above-mentioned range, the viscosity of the paint may increase and thus the flexibility of the paint film may be deteriorated, and the spray spraying may be lowered to deteriorate workability. On the other hand, when the above-mentioned range is exceeded, the hardness of the coating film may drop rapidly, and the coating film may be easily damaged by an external impact, and the viscosity may be lowered, so that the sagging resistance during spray painting may be sharply reduced.

flame retardant

In the epoxy fire resistant paint composition of the present invention, a phosphorus-based flame retardant may be used as the flame retardant. The phosphorus-based flame retardant serves to provide flexibility to the initial foamed carbonized layer by controlling the thermal decomposition rate of the cured coating film.

The phosphorus-based flame retardant may have a triphenyl phosphate (TPP) content of 20% by weight or less, and a thermal decomposition temperature of 250°C or higher (ie, a loss on heating of less than 5% at 250°C). The phosphorus-based flame retardant slows down the thermal decomposition rate of the cured coating film and imparts flexibility to the foam layer to prevent cracks from occurring in the carbonized layer, thereby ensuring long-term fire resistance performance without the use of a reinforcing material (mesh). When a phosphorus-based flame retardant with a TPP content exceeding 20% by weight and a thermal decomposition temperature of less than 250 ° C is used, the low molecular weight TPP vaporizes at a lower temperature than the cured epoxy resin to improve initial foaming. becomes larger, and long-term fire resistance performance may be reduced.

The phosphorus-based flame retardant is triphenyl phosphate (TPP), isopropylated triphenyl phosphate (isopropylated triphenyl phosphate), tricresyl phosphate (tricresyl phosphate), butylated triphenyl phosphate (butylated triphenyl phosphate), cresyl diphenyl aryl phosphates such as phosphate (cresyl diphenyl phosphate) and isopropyl phenyl diphenyl phosphate; at least one selected from bisphosphates such as resorcinol bis(diphenyl phosphate) (RDP) and bisphenol-A bis(diphenyphosphate) (BDP) can be used

For example, as the aryl phosphate, isopropylated triphenyl phosphate having a TPP content of 5 to 10% by weight may be used, and as the bisphosphate, RDP or BDP having a TPP content of less than 5% by weight may be used.

In the present invention, the phosphorus-based flame retardant may be a mixture of aryl phosphate and bisphosphate, and the mixing weight ratio may be 1: 0.5 to 3, for example, 1: 0.5 to 1.5.

The epoxy fire resistant paint composition of the present invention may include 5 to 20% by weight of the flame retardant, for example, 7 to 15% by weight, based on the total weight of the composition. If the content of the flame retardant is less than 5 wt%, the fire resistance performance may not be sufficiently exhibited. As the content increases, it may not be possible to secure the desired level of gas hazard reduction effect and fire resistance performance.

In the present invention, the mixing ratio of the epoxy resin, the reactive diluent and the flame retardant may be 100: 20 to 40: 50 to 70, for example, 100: 25 to 35: 55 to 65 by weight. When the mixing ratio of the epoxy resin, the reactive diluent, and the flame retardant is out of the above range, the carbonized layer may be easily cracked by a flame or lost by hot air, thereby reducing fire resistance.

hardener

The curing agent used in the epoxy fire resistant paint composition of the present invention is for curing the epoxy resin, and an amide or amido amine resin may be used as the curing agent.

A method for preparing the amide or amido amine resin is not particularly limited, and may be prepared by a method known in the art. For example, it can be prepared by polymerizing polyethylene amine, fatty acid dimer and fatty acid monomer having a viscosity of 200 to 800 cps (25 ° C.), an amine value of 300 to 600 mgKOH / g, and an active hydrogen equivalent of 50 to 200 g / eq. have.

For example, the temperature of the polyethylene amine, fatty acid dimer, and fatty acid monomer mixture is raised to 200° C. to perform a condensation reaction, and the reaction is performed until the acid value of the reactant becomes 1 to 5. In this step, the reaction can be made so that the molar ratio of amine/acid is 1.0 to 2.0. When the molar ratio is less than 1.0, the viscosity is high, so it may be difficult to apply the paint. Realization of physical properties may be difficult.

The polyethylene amine may be at least one of ethylenediamine, diethylenetriamine, triethylenetetraamine, and tetraethylenepentaamine. The fatty acid dimers and monomers may be monomers and dimers obtained from soybean oil fatty acid, tall oil fatty acid, castor oil fatty acid, rice bran oil fatty acid, linseed oil fatty acid, coconut oil fatty acid, lauric acid or linoleic acid, and these may be used alone or in combination of two or more. can be used For example, the fatty acid dimer and monomer may be a tall oil fatty acid monomer and dimer.

When the acid value of the reactant produced by the above reaction is 1 to 5 or less, heating is terminated to prepare an amide or amido amine resin.

The amide or amido amine resin has a viscosity of 200 to 800 cps (25° C.), for example, 300 to 600 cps (25° C.), and an amine value of 300 to 600 mgKOH/g, for example 400 to 500 mgKOH/g. and an active hydrogen equivalent of 50 to 200 g/eq, for example, 60 to 100 g/eq. When the amide or amido amine resin has physical properties within the above-described range, curability may be maximized.

The epoxy fire resistant paint composition of the present invention may include 10 to 15% by weight of the curing agent, for example, 11 to 14% by weight, based on the total weight of the composition. When the content of the curing agent is less than 10% by weight, adhesion of the coating film may be reduced, and when the content of the curing agent exceeds 15% by weight, defects in the coating film such as amine brushing may occur.

blowing agent

The epoxy fire resistant paint composition of the present invention may include a foaming agent. The foaming agent is exposed to high temperature heat and decomposes when the cured coating film is softened/liquefied and the carbonized layer is formed to generate a large amount of inert gas. The blowing agent may be melamine, urea, glycine, or a mixture thereof, but is not limited thereto.

The epoxy fire resistant paint composition of the present invention may include 3 to 10% by weight of the foaming agent, such as 5 to 10% by weight, based on the total weight of the composition. If the content of the foaming agent is less than 3% by weight, the paint may not expand sufficiently and thus the heat shielding performance may be deteriorated. may not work out.

acid catalyst

The epoxy fire resistant paint composition of the present invention may contain an acid catalyst. The acid catalyst is decomposed when the cured coating film is softened by exposure to high-temperature heat, thereby promoting the formation of the carbonized layer and accelerating the foaming of the carbonized layer. As the acid catalyst, ammonium phosphate, ammonium polyphosphate, melamine polyphosphate, melamine monophosphate, melamine bisphosphate, or a mixture thereof may be used, but is not limited thereto.

The epoxy fire resistant paint composition of the present invention may include 20 to 40% by weight of the acid catalyst, for example, 25 to 35% by weight, based on the total weight of the composition. If the content of the acid catalyst is less than 20% by weight, the fire resistance performance may not be sufficiently exhibited due to insufficient generation of the carbonized layer, and if it exceeds 40% by weight, the carbonized layer may be easily broken due to excessive foaming, thereby reducing the heat shielding performance.

fiber

The epoxy fire resistant paint composition of the present invention may include fibers. Fiber has a high specific surface area, so it provides thixo properties to the paint to prevent flow during painting, and prevents cracks by being evenly distributed in the cured film to act as a reinforcing material. It controls the foaming rate, reacts with the acid catalyst to supplement the strength of the foamed carbonized layer, and prevents cracks from occurring, thereby maintaining thermal insulation performance even at high temperatures.

Ceramic fiber, mineral wool, carbon fiber, Kevlar, or a mixture thereof may be used as the fiber, but is not limited thereto. Aluminosilicate fiber, magnesium silicate fiber, etc. may be used as the ceramic fiber, and synthetic glass (silicate) fiber, etc. may be used as the mineral wool, As the carbon fiber, a PAN based carbon fiber, etc. may be used, and as the Kevlar, a para-aramid synthetic fiber, etc. may be used.

The epoxy fire resistant paint composition of the present invention may include 3 to 10% by weight of the fiber, for example 3 to 8% by weight, based on the total weight of the composition. If the content of the fiber is less than 3% by weight, the effect of preventing cracks in the carbonized layer may be insufficient. This can fall.

additive

The epoxy fire resistant paint composition of the present invention may further include one or more additives selected from the group consisting of a dispersant, an antifoaming agent, a colorant, a thickener, a pigment, and a curing accelerator in order to exhibit optimal paint and coating film performance.

The additive may be appropriately added within a content range known in the art, for example, may be included in an amount of 0.01 to 20% by weight, respectively, based on the total weight of the epoxy fire resistant paint composition. When the content of the additive is within the above-described range, the appearance and hardness of the coating film may be improved.

The epoxy fire resistant paint composition of the present invention has a gas hazard of more than 12 minutes when measured by the gas hazard test method (KSF 2271), and the fire resistance performance when measured by the fire resistance test method (UL 1709) is 3 without the use of a separate reinforcing material (mesh) Over time, it has the effect of improving the release of harmful gases when the coating film expands in case of fire. Therefore, it is possible to reduce the risk of suffocation in the event of a fire, so it is effective as a fireproof paint for a closed space such as a tunnel or underground, as well as a facility handling oil or gas such as an offshore structure or a plant.

Hereinafter, the present invention will be described in more detail through examples. However, the following examples are only provided to help the understanding of the present invention, and the scope of the present invention is not limited to the examples in any sense.

[Example 1-11]

Each component was added according to Table 1 below, and dispersion in a bead mill was performed to prepare a main part and a curing agent part of Examples 1-11, respectively.

[Comparative Example 1-5]

Each component was added according to Table 2 below, and dispersion in a bead mill was performed to prepare a main part and a curing agent part of Comparative Examples 1-5, respectively.

Epoxy resin: Bisphenol A epoxy (Kukdo Chemical, YD-128, Mw 390 g/mol, epoxy equivalent 185 g/eq)

Reactive diluent 1: Trimethylolpropane triacrylate (Miwon Specialty Chemical, M-300, epoxy equivalent 100 g/eq, viscosity (25 ℃) 110 cps)

Reactive diluent 2: Neopentyl glycol diglycidyl ether (EVONIK, Epodil 749, epoxy equivalent 140 g/eq, viscosity (25 °C) 20 cps)

Reactive diluent 3: Neodecanoic acid 2,3-epoxypropyl ester (KCC, CRE00343, epoxy equivalent 245 g/eq, viscosity (25 °C) 7 cps)

Reactive diluent 4: diglycidyl ether of polypropylene glycol (EVONIK, EPODIL 777, epoxy equivalent 295 g/eq, viscosity (25 ℃) 40 cps)

Liquid Phosphorus-Based Flame Retardant: Triaryl Phosphates Isopropylated (LANXESS, Reofos 65)

Blowing agent: Melamine (SICHUAN GOLDEN ELEPHANT, JLS-Melamine)

Acid catalyst: Ammonium polyphosphate (JLS FLAME RETARDANTS, JLS-APP)

Dispersant: BYK, BYKW 980

Defoamer: BYK, BYK 085

Thickener 1: Organophilic phyllosilicates (BYK, GARAMITE 1958)

Thickener 2: Fumed Silica (KCC, D-200)

Toner: Carbon black (KCC)

Fiber 1: Aluminosilicate Fiber (UNIFRAX, HS-95C)

Fiber 2: Magnesium silicate fiber (UNIFRAX, HI-90)

Fiber 3: Mineral wool (LAPINUS, MS-615)

Fiber 4: Carbon Fiber (ACECA, 3NA)

Amine resin: polyamide (KCC, CRG00167; viscosity 450 cps (25 ° C), amine value 450 mgKOH/g, active hydrogen equivalent 120 g/eq, amine/acid molar ratio 1.1)

Cure accelerator: Tris-2,4,6-dimethylaminomethyl phenol (EVONIK, K-54)

Pigment: Titanium Dioxide (CHEMOURS, R-706)

[Experimental example: physical property evaluation]

The physical properties of the coating compositions of Examples and Comparative Examples were measured as follows, and the results are shown in Tables 3 and 4.

Fire resistance (UL1709)

It was measured according to the rapid heating oil fire test standard (UL1709, Rapid Rise Fire Tests of Protection Materials for Structural Steel). Based on the temperature of the specimen, at the end of the test, if the average temperature for each section is 538 ℃ or less and the maximum temperature for each sensor is 649 ℃ or less, the performance as a fire resistant structure is recognized. The fire resistance performance was measured with a test body (W10?ANSI Designation) coated with the paints of each Example and Comparative Example without using a reinforcing material (mesh free).

film elongation

It was measured according to ASTM D638 standard. After making a dumbbell-type specimen (Type I, film thickness 3.2±0.4 mm), the elongation was measured by pulling the specimen at a speed of 5 mm per minute.

[Unit: mm]

adherence

The coating composition prepared according to each Example and Comparative Example was applied to a thickness of 5 mm on a steel specimen (width 14 cm X length 14 cm X thickness 6 mm), and dried at room temperature for 7 days. Using a hole cutter, dig a hole enough to see the steel base, apply adhesive to the dolly and fix it on the specimen. After 4 hours, the adhesion strength was measured using a Pull-off Tester (Elcometer).

Sagging resistance

After the main part and the curing agent composition prepared according to each Example and Comparative Example were stored in an oven at 60° C. for 5 hours, respectively, the main part and the curing agent part were evenly mixed. The mixed paint composition was applied to the glass according to the thickness of the coating film using a sagging resistance applicator. Sagging resistance was measured by checking the thickness of the coating film that did not flow down when it was erected vertically immediately after application.

film hardness

A specimen (width 14cm X length 14cm X height 5mm) was prepared using the coating composition prepared according to each Example and Comparative Example, dried at room temperature for 7 days, and then the coating film hardness was measured using a Shore D hardness meter.

As can be seen from the results in Tables 3 and 4, the coating composition of Examples 1-11 according to the present invention exhibited excellent physical properties in all measurement items of fire resistance, coating film elongation, adhesion, sagging resistance, and coating film hardness. . In particular, it was confirmed that the coating composition of Examples 1-11 satisfies the long-term fire resistance performance of 3 hours or more and has excellent elastic modulus.

On the other hand, the paint compositions of Comparative Examples 1 and 5, in which the mixing ratio of the trifunctional or higher reactive diluent and the bifunctional or lower reactive diluent were outside the scope of the present invention, showed poor fire resistance performance. In addition, the paint composition of Comparative Example 2-4 using only one of a trifunctional or higher reactive diluent and a bifunctional or lower reactive diluent also showed poor fire resistance performance.

Claims (6)

an epoxy resin, a reactive diluent, a flame retardant, and a curing agent;
The reactive diluent is an epoxy fire resistant paint composition comprising a trifunctional or more reactive diluent and a bifunctional or less reactive diluent in a weight ratio of 80: 20 to 20: 80. The epoxy fire resistant paint composition according to claim 1, wherein the trifunctional or more reactive diluent has an epoxy equivalent of 70 to 130 g/eq and a viscosity of 60 to 140 cps (25° C.). The epoxy fire resistant paint composition according to claim 1, wherein the trifunctional or more reactive diluent is a trifunctional or more (meth)acrylate monomer. The epoxy fire resistant paint composition according to claim 1, wherein the difunctional or less reactive diluent has an epoxy equivalent of 100 to 350 g/eq and a viscosity of 5 to 50 cps (25° C.). According to claim 1, Based on the total weight of the epoxy fire resistant paint composition,
10 to 25% by weight of the epoxy resin,
2 to 20% by weight of the reactive diluent;
5 to 20% by weight of the flame retardant; and
10 to 15 wt% of the curing agent
An epoxy fire resistant paint composition comprising a. The epoxy fire resistant paint composition according to claim 1, wherein the mixing ratio of the epoxy resin, the reactive diluent and the flame retardant is 100: 20 to 40: 50 to 70 weight ratio.